A photodetector sensitive to the infrared wavelengths of light is also known as an infrared detector. Infrared detectors are used in a wide variety of applications, and in particular are used as thermal detection for surveillance, tracking, night vision, search and rescue, non destructive testing and gas analysis. Typically, an infrared detector is formed as a device consisting of an array, usually rectangular, of infrared light-sensing photodetectors disposed at the focal plane of an imaging lens. Such a detector is commonly referred to as a focal plane array (FPA).
Infrared covers a broad range of wavelengths, and many materials are only sensitive to a certain range of wavelengths. As a result, the infrared band is further divided into sub-bands such as near infrared defined conventionally as 0.75 to 1.0 μm; short-wavelength infrared (SWIR) defined conventionally as 1.0 to 3.0 μm; mid-wavelength infrared (MWIR) defined conventionally as 3 to 5 μm; and long-wavelength infrared (LWIR) defined conventionally as 8 to 14 μm. Infrared in the range of 5 to 8 μm is not transmitted well in the atmosphere and thus many mid-wavelength infrared detection applications operate within the 3 to 5 μm atmospheric window portion of the MWIR band.
Infra-red photon detectors are often produced using InSb and HgCdTe p-n junction diodes. However, these thermal detectors require cooling to cryogenic temperatures of around 77 K, which is complex, energy and volume consuming, and costly. The cryogenic temperatures are primarily used to reduce the dark current generated in the p-n junction diode in the bulk and at the surface by Shockley Reed Hall (SRH) generation, among other effects.
Photodetectors comprising a photo-absorbing layer, a barrier layer, and a contact layer have overcome many disadvantages of prior photodetectors, including mid-wavelength infrared detectors. A new class of photodetectors employing majority carrier filter principles is described in U.S. Pat. No. 7,687,871 to Maimon, filed Mar. 19, 2006, the entire contents of which are hereby incorporated by reference. These majority carrier filter photodetectors are often referred to as nBn detectors although the majority carrier filter principles may be employed using a variety of doping arrangements.
Photodetectors have been produced that are sensitive to a target waveband and that comprise a photo-absorbing layer preferably exhibiting a thickness of between one and two times the optical absorption length. These photodetectors may be comprised of an n-doped photo-absorbing layer, a barrier layer, and an n-doped contact layer. Other dopings may be used, such as p-doped photo-absorbing and contact layers as described by Maimon. These detectors may use an absorber to convert the incoming radiation into minority carriers which are collected to generate photocurrent. These detectors may use a barrier layer whose minority carrier band edge lines up with the absorber minority carrier band edge so that carriers can be collected. The majority carrier band edge of the barrier is well above the contact or absorber band edge such that majority carriers are blocked or filtered, thus producing the function described by the term “majority carrier filter.” The barrier layer exhibits a thickness sufficient to prevent tunneling of majority carriers from the photo-absorbing layer to the contact layer, and a barrier in the majority carrier energy band sufficient to block the flow of thermalized majority carriers from the photo-absorbing layer to the contact layer. The barrier layer does not significantly block minority carriers when appropriate bias voltage is applied.
In particular, for an n-doped photo-absorbing layer the heterojunction between the barrier layer and the absorbing layer is such that there is substantially zero valence band offset, i.e. the band gap difference appears almost exclusively in the conduction band offset. For a p-doped photo-absorbing layer the heterojunction between the barrier layer and the absorbing layer is such that there is substantially zero conduction band offset, i.e. the band gap difference appears almost exclusively in the valence band offset. Advantageously, these photodetectors can be operated with minimal to no depletion layer, and thus the dark current is significantly reduced. Furthermore, passivation is not required in arrayed photodetector elements as the barrier layer further functions to achieve passivation.
The specific materials used to produce a majority carrier filter are not critical so long as the valance and conduction bands are configured as described above. However, the materials should be selected to produce the valence and conduction band relationships discussed above. The barrier layer may comprise any suitable material such as one of AlSb, AlAsSb, GaAlAsSb, AlPSb, AlGaPSb and HgZnTe. Similarly, the photo-absorbing layer may be desirably constituted of one of n-doped InAs, n-doped InAsSb, n-doped InGaAs, n-doped InGaAsSb, n-doped Type II super lattice InAs/InGaSb and n-doped HgCdTe. The contact area may be constituted of one of InAs, InGaAs, InAsSb, InGaAsSb, Type II super lattice InAs/InGaSb, HgCdTe and GaSb. Alternatively, the photo-absorbing layer and/or contact layer may be p-doped. The contact layer and the photo-absorbing layer may exhibit substantially identical compositions. In the case where the photo-absorbing layer and contact layer have the same doping type but two different bandgaps, two-color operation can be achieved by reversing the bias voltage so that the photons absorbed in the “contact” layer are now collected in the “absorbing” layer. The bias can be alternatingly reversed so as to collect photons within two different radiation bands, corresponding to photons collected when the photodetector is forward-biased and when the photodetector is reverse-biased, respectively. For backside-illuminated focal plane arrays the top or “contact” layer should have the smaller bandgap in order to absorb the longer radiation band.
For a photo-absorbing material made from semiconductor materials, the absorption cutoff wavelength of the photo-absorbing material is generally determined by the composition of the semiconductor, but may be limited by dislocations in the molecular structure of the semiconductor lattice. The absorber material may be grown by liquid phase epitaxy (LPE), molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), or other methods known to those skilled in the art on substrate materials such as InSb, GaSb, InAs, InP, etc. However, to avoid substantial dislocations in the absorber material, the absorber should be a composition that has a crystal lattice constant similar to that of the substrate material. If the lattice constant of the absorber alloy does not match the lattice constant of the substrate material, the strain on the composition of the absorber due to the mismatch between the absorber and substrate lattice structures will increase as the absorber is grown, and will ultimately result in dislocations in the layers of the absorber lattice once the absorber exceeds a critical thickness.
For practical purposes, to grow an absorber with sufficient thickness such that the absorber has reasonable quantum efficiency, the lattice constant of the absorber must very closely match the lattice constant of the substrate upon which the absorber material is grown. This requirement that the absorber materials be lattice-matched to the substrate material effectively limits the absorption cutoff wavelengths of the photodetector to specific values. For example, an absorber comprising InAs(0.9)Sb(0.1), when lattice-matched to a substrate comprising GaSb and having a lattice constant of 6.09 A, exhibits an associated cutoff wavelength of 4.2 μm. However, this cutoff wavelength falls well short of the upper end of the 3 to 5 μm range for MWIR detection applications operating within the atmospheric window of the infrared band. Since there is much more infrared flux in the longer wavelength it is desirable to absorb the full MWIR band when the highest sensitivity for the photodetector is desired.
FIG. 2 shows the behavior of unstrained band-edge discontinuities as a function of the lattice constant. The unstrained band alignments of any two lattice-matched alloys can be determined by noting the relative position of their band edges in FIG. 2. For strained conditions, the mechanical strain fields alter the behavior of the electronic wavefunctions, thereby changing the valence and conduction bands, but the general behavior is similar. Highly ordered atomic transitions occur between lattice-matched semiconductor heterostructures with relatively little atomic and electronic reconstruction. However, in a lattice-mismatched condition defects occur in the crystal structure of the absorber when the absorber lattice dislocates to relieve the excess strain resulting from the lattice mismatch between the absorber and the substrate. These crystalline defects directly effect the operating characteristics of the photodetector device, degrading the quality, e.g. the radiative efficiency and thermal noise, of the semiconductor.
There are limited substrates available for crystal growth, usually the binary materials like InAs or GaSb shown in FIG. 2 although other alloys may be used. Conventionally, the only cutoff wavelengths available come from alloys whose lattice constant match the substrate. It is desirable for design versatility in various infrared sensing applications to be able to achieve cutoff wavelengths other than and beyond those of lattice-matched systems. Sometimes it is possible to grow quaternaries to raise or lower the band gap, for example, by adding aluminum or nitrogen to the absorber material. However, this approach can result in poor absorber material quality or a complicated growth processes.
Additionally, despite the potential for lattice defects, photodetectors with highly strained absorbers have been grown to achieve an extended cutoff frequency. For example, the maximum wavelength normally achievable with a highly strained absorber on a substrate comprising GaSb is 4.5 μm or less. However, the high strain resulting from this approach limits the absorber thickness and material quality. Superlattices using InAs/GaSb have been used to create an effective tunable bandgap in the GaSb system. However, the complicated growth and the band edges of the superlattice system relative to the barrier have made it difficult to achieve MWIR barrier detectors with superlattice absorbers.
It is desirable for photodetectors, and in particular majority carrier filter type photodetectors with reduced dark current, to operate in wavelengths not accessible using materials in the lattice-matched condition. Aspects of the present invention relate generally to solving the problems associated with photodetectors, in particular majority carrier filter type photodetectors, where absorption cutoff wavelengths are limited by absorber/substrate lattice-matching requirements, such that photodetectors exhibiting cutoff wavelengths beyond conventional cutoff wavelengths can be realized. It is desirable to operate in wavelengths not accessible in the lattice-matched condition and to extend the wavelength out to 5.0 μm or further to take full advantage of the 3 to 5 μm atmospheric transmission portion of the MWIR band. The present invention has been developed in view of these considerations, and therefore it is an object of the invention to provide a photodetector with an extended cutoff while also maintaining the band lineups at the junctions of the layers of majority carrier filter detector.